1. Field of the Invention
The present invention relates to a fusing roller of an image forming apparatus.
2. Description of the Related Art
A general electrophotographying image forming apparatus using an electrophotographic developing method, such as a photocopier, a laser printer, and the like, has a photosensitive medium that is electrically charged at a predetermined level by a charging means. One method by which the photosensitive medium is electrically charged by the charging means is by exposing the photosensitive medium to a laser beam projected from a laser scanning unit in a predetermined pattern. As a result, an electrostatic latent image is formed on a surface of the photosensitive medium. A developer supplies a toner to the photosensitive medium to thus develop the electrostatic latent image formed on the photosensitive medium to a toner image, which is in a powdery state and visible. Then, when a printing paper passes between a photosensitive drum and a transfer roller rotating in contact with the photosensitive drum, the toner image of the photosensitive medium is transferred to the printing paper. A fusing unit, including a fusing roller, fuses the transferred toner image onto the printing paper with a predetermined pressure and a predetermined temperature. Through this fusing process, the toner image is finally printed on the printing paper. As described above, the fusing unit for fusing the toner image onto the printing paper includes the fusing roller rotating in contact with a backup roller at a high temperature.
Generally, the fusing roller is structured in a manner so that a heat pipe is disposed inside a metallic pipe. An operating fluid contained in the heat pipe is heated by the driving of a resistance heating body, thereby enabling heat to be transmitted to the metallic pipe. This increases the temperature of the metallic pipe to the previously discussed operating temperature within a predetermined time. However, since the heat of the heat pipe disposed inside the metallic pipe is transmitted to the metallic pipe in a non-contacting manner, there is a disadvantage in that it takes a long time to heat the metallic pipe. As a result, the FPOT (First Print Out Time), the delay before the printed image is produced, becomes longer.
In order to solve this problem, there have been recent suggestions of a fusing roller that is capable of directly transmitting heat generated at a resistance heating body to a metallic pipe, so as to heat the metallic pipe within a short time. As for such a fusing roller, there is known in the art a low current/low voltage direct heating instant fusing roller. In this case, the resistance heating body is interposed between an outer metallic pipe and an inner metallic pipe, the inner and outer metallic pipes having different diameters, so that the outer metallic pipe is directly heated by a resistance heat of the resistance heating body. However, an insulator is required for insulation between the resistance heating body and the metallic pipes. An electric insulator, generally having a low degree of heat conduction, deteriorates thermal transmission efficiency, causing the FPOT to increase. Accordingly, improvements in the material and thickness of the electric insulator are required to shorten the FPOT, while still satisfying insulation standards for safety.